1. Field of the Invention
The present invention relates to a method and apparatus for continuous finishing hot-rolling steel strips.
2. Description of the Related Art
In conventional processes for making steel strips, a slab fed from continuous casting equipment is cut to a given length. Each cut slab is rolled by a rough hot-rolling mill and then by a finishing hot-rolling mill to form steel strips. The steel strips are subsequently coiled.
Recent patents and other documents disclose a method for continuously performing rough and finishing hot-rolling steps in order to improve the quality and the yield of the steel strip, to save energy, and to improve productivity. However, the conventional continuous hot-rolling methods are not practical.
Cold rolling steps have also been used for continuously rolling steel strips. For example, Japanese Laid-Open Patent No. 4-339501 discloses a work roll cross rolling method for a joint of a strip in a continuous cold-rolling method. The reduction ratio and the roll cross angle of a roll incorporated in a stand of rolling mill are changed at the joint of the strip. The crown ratio to the preceding strip is changed at the joint of the strip since it assumes that the difference of the strip thickness between the preceding strip and the succeeding strip is large.
Japanese Laid-Open Patent No. 4-351213 discloses a timing of instructing the changes in the roll cross angle and roll bending force during cold rolling coils which are formed continuously by means of joining. However, timing of change is only carried out by detecting the joint position. Information regarding the physical properties of the joint is not taken into account during the rolling process.
In Japanese Laid-Open Patent No. 52-86956, the rolling speed is reduced when the joint is inserted between the work rolls of the first stand in continuous cold rolling mill. Further, in Japanese Laid-Open Patent No. 60-206505, the tension of the strip is controlled between stands.
However, as opposed to the above-described cold rolling methods, when continuous rolling is carried out with hot rolling equipment, the temperature of the steel strip abruptly increases near the joint. Thus, resistance against deformation at the joint becomes non-uniform and abruptly decreases. These phenomena do not occur in the continuous cold rolling. Therefore, in the hot rolling process, speed control is required at the joint. Also, the joint must be passed through under a controlled rolling reduction ratio and a controlled crown without stress concentration. Each rotation speed of mill rolls of stands must be synchronized with the response of the rotation speed control so that the tension of the strip and the loop do not abruptly change.
Further, a temperature difference exists between the non-joint section and at a location near the joint in the strip. The method for finishing the strip having such a non-uniform temperature distribution to a uniform strip profile has not been completed. Moreover, no prior continuous rolling method discloses that materials with different thicknesses, widths and qualities are joined to each other.
Japanese Laid-Open Patent No. 6-39404 discloses a method for preventing the breakage of the sheet bar in exclusively continuous hot rolling by strengthening the joint of the sheet bar. The butts of the sheet bars are rolled while adjusting the crown schedule with the first and second stands prior to the finishing rolling mill. However, a method for controlling the inlet side stand of the finishing rolling mill to prevent such a breakage is not disclosed.
Japanese Laid-Open Patent No. 60-244401 discloses a method and apparatus for hot-rolling a slab after heating and press-joining the rear end of the preceding slab and the leading end of the succeeding slab. However, the hot rolling process itself is not disclosed in detail.
Japanese Laid-Open Patent No. 63-90302 discloses a method for hot-rolling the slab after heating and press-joining the rear end of the preceding slab and the leading end of the succeeding slab on a travelling carriage. As with Japanese Laid-Open Patent No. 60-244401, the hot rolling process itself is not disclosed in detail. Additionally, in such a conventional continuous hot rolling process, the joint ruptures at the first stand of the finishing hot rolling mill.
Methods for preventing the joint from rupturing in the finishing hot rolling mill include, increasing the rigidity of the rolling mill (Japanese Laid-Open Patent No. 7-16607), and smoothing the temperature distribution around the joint (Japanese Laid-Open Patent No. 7-16611). These methods prevent the joint from breaking by suppressing the tension variation between stands when rolling the joint. Thus, these methods are effective in preventing the joint from breaking at the second half stage of the finishing hot rolling mill. However, these methods do not solve the problems set forth above. Further, since the strip bars are joined before continuous hot-rolling, the work rolls and the motors for driving the work rolls are overloaded due to decreased mill stoppage time. The industry lacks a method and apparatus which addresses the problems described above.